Nerve terminals are richly endowed with presynaptic receptors for a variety of transmitters, and activation of some classes of these receptors are well-known to mediate the actions of diverse classes of drugs. In the central auditory system, however, the functions of these receptors is not well understood. We propose to take advantage of an accessible and well-studied auditory synapse, the calyx of Held, to explore the mechanisms of action, the modulation, and the physiological functions of several novel presynaptic receptors. Most attention will be given to the presynaptic glycine receptor (GlyR), which mediates an increase in the release of the excitatory transmitter glutamate by depolarizing the calyceal nerve terminal. In experiments using patch clamp techniques we will contrast the biophysical properties of these receptors in calyces and in postsynaptic cells to gain evidence of subunit composition. Additionally, we will determine the properties of these receptors in mouse mutants having defects in specific subunits of the receptor. Other experiments will determine what second messenger pathways modulate these receptors and how activation of these receptors function in concert with their better known postsynaptic counterparts. Previous data indicate that the GABAA receptors are down regulated upon expression of the GlyR, suggesting a coupling in the mechanisms of their expression or targeting. In developmental studies we therefore will examine the coordinate expression of presynaptic GABAA receptors and GlyR in nerve terminals of the mutant mice, to determine if loss of GlyR function has consequences for GABAergic transmission. Finally, we will explore the properties of an NMDA response in the calyx terminal, which appears to have properties distinct from conventional postsynaptic NMDA receptors. Together, these studies should significantly expand out understanding of synaptic interactions in the auditory brainstem and potentially identify new drug targets for selective control of processing of auditory signals.